The present disclosure is directed to the manufacture of inserts, and more particularly directed to the fabrication of wear resistant cobalt alloy inserts using various sintering techniques including microwave radiation. Inserts are typically installed in drill bits for drilling an oil well.
An oil well is drilled with a typical tricone drill bit and assembly with threads to the bottom of a string of drill pipe. It has a hollow threaded member with an axial flow passage within the assembly to direct drilling fluid, usually known as drilling mud, out through a number of openings to wash cuttings away from the cones which form the cutting. Rotation of the drill string and attached drill bit is from the surface of the earth. Teeth on the drill bit are rotated against the face and wall of the well borehole thereby cutting the earth formations as the drill bit rotates, thereby advancing the borehole. The drill bit has three cones mounted for contact against the face of the borehole. Each cone rotates its teeth with the rotation of the drill string, thereby cutting the borehole. Drill bit wear predominately occurs at the teeth. As the teeth wear, the penetration rate declines and the drill bit has to be replaced.
Cones and their teeth have a specified wear rate. Better performance has been obtained by enhancing the wear characteristics of the cone teeth, or xe2x80x9cinsertsxe2x80x9d. Inserts are positioned within each cone hole. The inserts are harder than the metal cone. Most inserts are formed of various carbides, extremely hard materials. Primary contact and wear of the insert occurs at the exposed outer end of the insert. Greater protection yet has been provided from industrial grade diamonds. The optimum wear protection is obtained by the attachment of a cap or crown of industrial grade diamond which covers the exposed insert end. This type of crown is often known as a polycrystalline diamond compact (PDC). The carbide insert body is not pure WC, but is preferably granules of WC which are interspersed with an alloy which binds the WC particles. The preferred alloy is a cobalt based alloy. Likewise, the PDC crown is not a layer of pure diamond, but is an agglomeration of diamond particles held together with a binding metal matrix. Again, this binding material is typically a cobalt based alloy. The PDC cap or crown is normally attached to the WC insert body by ultra high pressure and heat. The sintering material may also contain a substantial amount of cobalt. Specific materials are notable. The insert body is usually WC which is harder than other common metal carbides. While other metal carbides will work in some degree, WC is the common and preferred material. In like fashion, the binding alloy is usually about 15% or so of cobalt in the alloy matrix holding the WC particles together. A common alloy with WC is sold as the model 374 by Roger""s Tool Works and includes an alloy having as low as 6% up to about 15% cobalt with other metals of less significance. The cobalt is the most significant part of the alloy as will be discussed below.
In prior art, elements of the insert are typically manufactured separately and subsequently assembled. The manufacture of the components is usually by sintering under very high temperature and very high pressure. This requires equipment which is physically large, and which is also very expensive to manufacture, maintain and operate. In addition, the high temperature can induce adverse chemical and physical changes in insert components, which will be discussed in subsequent sections of this disclosure.
As discussed in U.S. Pat. No. 5,011,515, composite polycrystalline diamond compacts, PDC, have been used for industrial applications including rock drilling and metal machining for many years. As an example, the composite compact consisting of PDC and sintered substrate are affixed as insert elements in a rock drill bit structure. One of the factors limiting the success of PDC is the strength of the bond between the polycrystalline diamond layer and a sintered metal carbide substrate. It is taught that both the PDC and the supporting sintered metal support substrate must be exposed to high pressure and high temperature, for a relatively long period of time, in order to achieve the desired hardness of the PDC surface and the desired strength in the bond between the PDC and the support substrate.
U.S. Pat. No. 3,745,623 (reissue U.S. Pat. No. 32,380) teaches the attachment of diamond to tungsten carbide (WC) support material with an abrupt transition there between. This, however, results in a cutting tool with a relatively low impact resistance. Due to the differences in the thermal expansion of diamond in the PDC layer and the binder metal alloy used to cement the metal carbide substrate, there exists a shear stress in excess of 200,000 psi between these two layers. The force exerted by this stress must be overcome by the extremely thin layer of cobalt which is the common or preferred binding medium that holds the PDC layer to the metal carbide substrate. Because of the very high stress between the two layers which have a flat and relatively narrow transition zone, it is relatively easy for the compact to delaminate in this area upon impact. Additionally, it has been known that delamination can also occur on heating or other disturbances in addition to impact. In fact, parts have delaminated without any known provocation, most probably as a result of a defect within the interface or body of the PDC which initiates a crack and results in catastrophic failure. See also U.S. Pat. No. 4,811,801.
One solution to the PDC-substrate binding problem is proposed in the teaching of U.S. Pat. No. 4,604,106. This patent utilizes one or more transitional layers incorporating powdered mixtures with various percentages of diamond, tungsten carbide, and cobalt to distribute the stress caused by the difference in thermal expansion over a larger area. A problem with this solution is that xe2x80x9csweep-throughxe2x80x9d of the metallic catalyst sintering agent is impeded by the free cobalt and the cobalt cemented carbide in the mixture. In addition, as in previous referenced methods and apparatus, high temperatures and high pressures are required for a relatively long time period in order to obtain the assembly disclosed in U.S. Pat. No, 4,604,106. Pressures and temperatures are such that, using mixtures specified, the adjacent diamond crystals are bonded together.
U.S. Pat. No. 4,784,023 teaches the grooving of polycrystalline diamond substrates but it does not teach the use of patterned substrates designed to uniformly reduce the stress between the polycrystalline diamond layer and the substrate support layer. In fact, this patent specifically mentions the use of undercut (or dovetail) portions of substrate ridges, which solution actually contributes to increased localized stress. Instead of reducing the stress between the polycrystalline diamond layer and the metallic substrate, this actually makes the situation much worse. This is because the larger volume of metal at the top of the ridge will expand and contract during temperature cycles to a greater extent than the polycrystalline diamond, causing the composite to fracture at the interface. As a result, construction of a polycrystalline diamond cutter following the teachings provided by U.S. Pat. No. 4,784,023 is not suitable for cutting applications where repeated high impact forces are encountered, such as in percussive drilling, nor in applications where extreme thermal shock is a consideration.
By design, all of the cutting surfaces consisting of xe2x80x9cconventionalxe2x80x9d alloys which are disclosed in the above references are xe2x80x9chardxe2x80x9d in that they are abrasion and erosion resistant. This is particularly true for PDC material which is also quite brittle and subject to fracturing upon impact. Because of the brittleness and overall hardness, it is not practical and economical to machine surfaces of tools, bearings and the like made of PDC in the manufacturing process for these devices. Alternately, the PDC surfaces are preferably xe2x80x9cmoldedxe2x80x9d or performed using techniques taught in U.S. Pat. No. 4,662,896. Brittleness and fracture resistance are also noted in U.S. Pat. 4,811,801.
The paper xe2x80x9cIron Aluminum-Titanium Carbide Composites by Pressureless Melt Infiltration-Microstructure and Mechanical Propertiesxe2x80x9d by R. Subramanian et al (Scripta Materialia, Vol. 35, No. 5, pp. 583-588, 1996, Elsevier Science Ltd.) discloses a technique for fabricating wear resistant material which does not require high pressure. Conversely, a mixture of powdered components is placed in a dynamic vacuum of 10xe2x88x924 Pa, heated to a temperature of 1450 for about one hour. The binding component melts and flows into the interstitial voids of the wear resistant component. Vacuum equipment is obviously required to fabricate the wear resistant material.
U.S. patent application Ser. No. 08/517,814 which was filed on Aug. 22, 1995 by the present inventor discloses apparatus and methods for forming composite inserts at relatively low temperature and pressure. The composite insert can be assembled by brazing a separately sintered wear component to a support component, or by sintering the wear component directly onto the support component. The wear surface consists of a sintered mixture or xe2x80x9ccermetxe2x80x9d of crystalline material, metal and/or metallic carbides. These alloy materials are selected to minimize the sintering heat and temperature requirements. In a preferred embodiment, the wear surface material created by sintering consists of a mixture of abrasion resistant crystals, preferably diamond crystals, and a metal, which partially transforms during sintering to metal carbide, is a cemented diamond compact containing 60% or more diamond by volume, but lacking diamond to diamond bonding. Due to the high metal content and the short time of sintering, not all of the metal is reacted with the abrasion resistant material. The metal which is not reacted is then free to form a matrix in which the abrasion resistant material is suspended. This metal matrix is responsible for the enhanced ductility and fracture toughness of the material. The end result is a material with comparable abrasion and erosion properties to conventional, prior art materials, but the cermet is less costly to produce, has better impact resistance, and is more easily formed. A mold or cast is required to contain the wear resistant component in the low temperature cermet alloy during the low temperature and low pressure sintering operation. Disclosed means for heating are a simple torch, an induction oven, a source of infrared light, a laser source, a plasma, or a resistive heating oven. Attempts are made to use materials with matching thermal coefficients to minimize stress between the cermet and support components and stress within the cermet, although it is still sometime preferable to anneal the final product to reduce stress in the finished product.
The parent application for this continuation-in-part discloses apparatus and methods for forming sintered components of alloys using microwave energy as a heat source, wherein the alloys are xe2x80x9cconventionalxe2x80x9d in that they were previously used only in high temperature and high pressure sintering processes The insert body and the insert wear crown can be sintered as an integral insert within a mold, or can be sintered separately and subsequently joined by brazing as previously discussed. As an important additional advantage, the mold to contain the raw materials can even be completely eliminated by the use of a sacrificial binding agent such as wax prior to sintering. The microwave energy source permits the sintering process to be completed in a relatively short period of time, and at very low pressure. Temperature can also be controlled. If sintered as a unit, migration of cobalt within the various components is negligible due to the relatively short sintering time required. The disclosure also teaches that smaller grain sizes can be obtained without the use of grain growth inhibitor, which can adversely affect the insert in other ways. Stress concentration at the interface of insert components is still present, although markedly reduced if the insert is sintered as a unit. Stress concentration at the interface of components assembled after sintering can be significant.
There is a delicate balance to be obtained in the finished wear product between hardness and resiliency. If materials are harder, they are lacking in resilience, and if they are resilient, they are lacking in hardness. As discussed previously, composite materials such as a wear resistant crown and an insert body of differing material yield high quality inserts. However, the composite materials are all different and therefore have contradictory criteria meaning they have different measures of hardness, different resiliency, different rates of thermal expansion, and different measures of shock resistance. A representative insert will be described which utilizes a central steel shank or body. The body, in turn, is covered with the WC abrasive resistant material. Separately, a PDC crown is made at another location and then this PDC layer is brazed to the partly finished WC clad steel shank. Prior art manufacturing is typically by high pressure high and temperature sintering, sometimes known as xe2x80x9cHPHTxe2x80x9d sintering. While the finished product is quite successful, there are, however, problems that arise because of the dissimilarities in the various materials making up the finished device. In one aspect, the sintering process mandates that the components be made separately and later joined. This leads inevitably to transverse planar regions which localize possible stress failure. In a typical insert, the PDC crown is brazed by a braze region which measures only about 0.001 to about 0.004 inches thick. Moreover, this thin region of braze material must secure dissimilar materials together so that there are stress levels in this braze region which are detrimental to long life. Even if the stress is relatively minimal by careful manufacture, the drill bit is used in elevated temperatures so that stress concentrations can again build up which are not common at ambient temperatures. Regrettably, the failure mode of many inserts is fracture along the braze plane so that part or all of the PDC crown will break off.
This type of insert defies stress relieving by annealing using some prior art teachings. For instance, in the manufacture of glass and other relatively brittle materials, the finished product can be gently heated to a relatively high temperature for a long period of time and then gently cooled over a long time interval to obtain some internal stress relief. That is not so readily effective for composite drill bit inserts. There is a problem with migration of cobalt between differing elements or regions of the composite insert. Suffice it to say, the cobalt levels in different regions vary because different quantities of cobalt are required to provide the bonding matrix holding the various different particles together. The cobalt concentration in the PDC layer is different from the cobalt concentration in the braze layer, and is different from that in the WC sheath. Heating for a long interval at elevated temperature may enable the cobalt concentration to simply average out, thereby degrading the performance of the cobalt based alloy in one region or the other.
The heating phase of both sintering manufacturing methods and post manufacture annealing methods can also be detrimental to the different regions of the insert. As an example, the crystalline structure of carbon on the PDC can be adversely affected by physical changes at high temperatures, whether applied in the manufacturing step or the annealing step. This reduces the wear properties of the PDC. Above a certain temperature, the carbon will begin to oxidize or otherwise be affected chemically, thereby also significantly reducing the wear properties of the PDC. Therefore, it is necessary to maintain sintering and annealing temperatures below a threshold at which damage to the PDC is incurred. Using prior art teaching, this can be accomplished by longer sintering and annealing heating times but at lower temperatures. These longer heating periods, however, result the previously discussed cobalt migration problem which, contradictorily, is minimized by heating for a shorter period of time but at a higher temperature.
Sintering and annealing at elevated temperatures for long periods of time can be detrimental to the grain size of the wear surface which can, in turn, affect the resilience of the wear surface. The smaller the grain size, the more resistant the material is to chipping and fracturing. High sintering and annealing temperatures tend to increase the grain size of sintered material and thereby degrade wear properties.
The use of a mold to fabricate wear inserts or integral wear resistant parts can be very expensive, especially if relatively small numbers of pieces are to be fabricated. An expensive mold or cast is required in the sintering of conventional alloys using high temperature-high pressure techniques while a low cost mold is need in microwave sintering of conventional alloys using methods and apparatus disclosed in the parent U.S. Patent Application.
In summary, prior art teaches the manufacture and the use of various abrasion and erosion resistant materials to form inserts which are used as wear surfaces in drill bits, and which can also be used for wear surfaces on machine tools, drill bits, bearings, and other similar surfaces. Many of the processes in the cited references require high temperatures and high pressures to sinter conventional alloys for a relatively long period of time to form the wear resistant surface material, or to bond the wear resistant surface material to the underlying support substrate, or both. A mold or cast is required. Using a composite drill bit insert as an example, cobalt can migrate between wear surface, braze layer, and insert body thereby perturbing the desired concentration of cobalt in each element of the insert. Furthermore, the bond between surface and substrate of the resulting inserts is subject to weakening due to differences in thermal expansion properties which become a factor as the device heats up during use. This can be reduced by annealing, but annealing at high temperatures over long periods of time also -results in cobalt migration as discussed in the example above. Sintering and annealing heating for extended periods of time can also cause grain size growth which yields a wear surface which is quite brittle, subject to fracturing upon impact, and are in general very difficult to handle in the manufacturing process of tools employing such wear resistant surfaces. Sintering and annealing at high temperature can also adversely affect the chemical and physical properties of the wear surface. As an example, a PDC wear surface will tend to oxidize if heated at elevated temperatures. To minimize elemental migration between regions, to minimize grain growth, and to minimize damage to the wear surface, it is desirable to apply sintering and annealing heat at a relatively low temperature and for a relatively short period of time. Low pressure is also desirable from an economic and operational point of view. Low pressure and low temperature sintering of wear resistant components enable a low temperature allow and a mold or cast to be used. The fabrication of wear elements by means of low temperature-low pressure sintering of conventional and low temperature alloys, using microwave energy, without the use of a mold, are not known in the prior art.
The present invention sets out an improved alloy system with different levels of key ingredients in different regions. When bonded by heat, alloy migration in the regions is prevented, and regional differences are preserved. This enables simultaneous bonding of a PDC layer with a higher level of cobalt, an amount usually around 15% cobalt.
The WC body of the insert is alloyed with cobalt; but contrary to prior WC alloy bonding, the cobalt is not 15% or so. Rather it is in the range of about 6 to 10% cobalt. The optimum for many WC insert bodies is around 8% cobalt. The process begins with the PDC and WC ingredients in a mold compressed by packing with light pressure. The loose molded ingredients are held in the mold with minimal pressure prior to heating.
Microwave heating is preferred because it is quicker, operates at a lower temperature, and needs only minimal or no pressure, and can be done in a low pressure mold.
One object of the invention is to provide apparatus and methods for manufacturing sintered, composite wear inserts, wherein the sintering temperature is generated by microwave energy and is below a level which inflicts adverse physical and chemical changes in components of the composite insert.
Yet another object of the invention is to provide apparatus and methods for manufacturing sintered, composite wear inserts, wherein the heating cycle is relatively short in duration thereby preventing elemental migration between various components of the composite insert.
Still another object of the invention is to provide apparatus and methods for manufacturing sintered, composite wear surfaces, wherein the magnitude and duration of the heating phase of the sintering operation is set to minimize grain size growth in components of the composite insert.
An additional object of the invention is to provide apparatus and methods for effectively sintering low cobalt insert bodies. One benefit of the approach is reducing stress concentration at component interfaces, minimizing the migration of constituents between the components, and inhibiting grain growth within the components.
A still further object of the invention is to provide apparatus and methods for fabricating wear elements without the use of a high pressure cast or mold.
The present disclosure is summarized as a method for sintering composite wear inserts using microwave radiation as a heat source. Low cobalt or low temperature alloys can be used in the wear inserts, and a simple mold or cast is used for the fabrication process.
As a precursor to summarizing the invention, the basic principles of interaction of microwave radiation with metal will be reviewed. The modes of interaction between material and electromagnetic radiation in the microwave region can be defined as transparent, absorbent and reflective. The interaction is defined as transparent when the microwave radiation passes through the material with little attenuation. The interaction is described as absorbent when the microwave radiation is completely absorbed within the material. The interaction is described as reflective when the microwave radiation is reflected away from the material without attenuation.
The modes of interaction between microwave radiation and material are affected by the frequency of the radiation and the temperature of the material. Assume first that for a given material temperature, the mode of interaction is reflective. As the frequency of the radiation is changed to some threshold level, some of the microwave radiation will be absorbed by the material. As the frequency is further altered, more radiation will be absorbed. Eventually a frequency will be reached at which all radiation will be absorbed. If the frequency is still further changed, absorption will decrease and transparency will become a mode of interaction. When the frequency is changed beyond a second threshold level, the material will become completely transparent.
Assume again that for a given material, the mode of interaction is reflective. Further assume that the frequency of the microwave radiation is held constant. As the material is heated (presumably from an external source) above a threshold temperature level, the dielectric loss begins to increase rapidly and the material begins to absorb microwave radiation and reflect less. The absorption also generates heat to rapidly increase the temperature of the material internally and independent of any external heat source. As the temperature of the material is increased further, absorption dominates the interaction mode and as the temperature is increased even further (presumably by means of an external heat source), absorption declines and reflection dominates.
In the remaining portions of this disclosure, it will be assumed that all microwave sintering and stress relieving processes begin at an ambient xe2x80x9croom temperaturexe2x80x9d.
Turning first to the manufacture embodiment of the invention, microwave heating has demonstrated itself to be a powerful technique for sintering various ceramics, especially through the past decade. Microwave heating may decrease the sintering temperatures and times dramatically, and is economically advantageous due to considerable energy savings. However, one of the major limitations is the volume and/or size of the ceramic products that can be microwave sintered because of non homogeneous microwave energy distribution inside the applicator which often results in a non-uniform heating.
This disclosure features two of three different types of products of manufacture which can be handled by microwave heating to obtain sintering. The three different types of products refers to the form of the products, not the chemical makeup of the products. Indeed, the products can be made of the same constituent ingredients. They differ however primarily in the shape and hence the cohesive nature of the respective products. These three product formats or forms include loose particulate material such as (1) a powder of a specified size, (2) a molded product, or (3) a precast molded product. The distinction in the latter is that it is precast sufficiently that it requires no mold during sintering. It can be precast with a sacrificial wax, adhesive, moisture are even low pressure compaction of the material which forms the particles together into a desired precast form. During sintering, the form is not changed in terms of shape, but the form is sustained although this is accomplished free or devoid of a confining mold. The molded product is a product which is held in a mold during sintering. One of the advantageous aspects of the molded products is that initial mold shaping of the particles making up the product can be accomplished at very low temperatures and pressures, i.e., substantially at room temperature and atmospheric pressure. Typically, loose particles are joined in a mold again by a sacrificial wax, other material, low pressure compaction or alternately by the confines of the cavity mold itself. In either instance, the finished product is a structure which is sintered and yet which has a defined shape or profile. Examples abound as will be set forth below.
In all instances, all examples will be described so that the sintering process begins or acts on what are known as xe2x80x9cgreenxe2x80x9d materials. The term xe2x80x9cgreenxe2x80x9d materials refers to those materials which have been provided but have not been sintered. These green materials are the low temperature-low pressure alloys disclosed in the parent U.S. Patent Application. In addition, the green materials can consist of conventional ingredients used in prior art high pressure-high temperature sintering techniques taught in the prior art. For particulate matter, the green materials typically have the form of powders. Both in the molded and precast forms, one of the beginning materials is the requisite quantity of particles prior to molding, i.e., shaping into a desired form either by precast molding or sintering in a mold.
The preparation of loose material to be sintered defines small particles which can be used later in a wear surface and the like. Normally, these materials must be sintered to a specified grain size. In many applications, the quality or performance of the material is directly impacted by the grain size accomplished in the sintering process. In one aspect, grain size has an undesirable impact on the finished product. More specifically, this arises from the fact that additives often are placed in controlled quantities in the material prior to sintering so that the grain boundaries are defined by the additives. While there are additives available which do control grain size, the additives weaken or reduce the hardness of the finished product. Therefore such additives, while desirable in one aspect, are not desirable in other regards. The amount, nature, and dispersal of such grain boundary additives is a material factor, thereby providing a balanced mix of properties where the properties themselves result in some kind of compromise in the design of such sintered products. Effectively, grain boundary size is controlled only at a cost in sintered particle hardness.
Continuous microwave sintering is designed to focus the microwave radiation field in a central area as uniformly as possible. A long cylindrical ceramic hollow tube contains the unsintered (or green) material which is fed into the microwave applicator and into the central area at a constant feed speed. As the green material enters the microwave cavity, it is heated and gradually sintered while passing through the microwave zone. The heating rate, sintering time and cooling rate are controlled by the input microwave power, the feeding speed, and the thermal insulation surrounding the heated material. The ceramic hollow tube can also be rotated during processing for more uniform and homogeneous heating. As the green material passes through the high temperature zone, the particles are sintered entirely. Since the ceramic hollow tube is moved continuously in the axial direction during the processing, there is virtually no limitation to the length or volume of the product that can be processed by this technique. Consequently, it is possible to scale up the volume of the ceramic products to be microwave sintered by this technique by implementing a continuous process.
This disclosure proves the continuous microwave sintering for drill bit inserts. The results show better physical properties than the conventionally processed material. The disclosure sets out two different product configurations. One form is a cold press shaped or configured particulate body shaped by a mold at minimal pressure, and a third form is a cold pressed, unconfined form of sufficient strength to hold its own shape either with or without a sacrificial binding agent such as wax. The products are generally referred to below as molded products and precast products.
In prior art devices, molds are typically used for sintered particles or for composite cast items (molded or precast) such as wear inserts for drill bits. A molded part can be sintered by placing green particulate materials in a mold or cavity in the desired geometric configuration. The mold is first filled with the appropriate, configured green constituent materials. As an example, tungsten carbide or silicon nitride particles are packed into a mold or cavity. An interspersed particulate binder metal, typically a cobalt alloy, is added in the mold or cavity. In the prior art, extreme heat with deleterious consequences was applied in the ordinary manufacturing process along with extremely high pressure to form a molded part. The resultant part is a matrix of hard particles which are held together by the melted alloy. The alloy serves as a binder which holds the shape of the finished part. By applying an adequate high pressure to the cavity and by also applying an adequate high temperature for an adequate interval, molded parts were made in this fashion. The prior art high pressure and high temperature (HPHT) equipment is quite large, quite expensive to fabricate, and quite expensive to operate. Furthermore, high temperature and/or extended heating periods can be detrimental to the final product as discussed previously.
The microwave process of this disclosure does not require massive and expensive manufacturing equipment, thereby reducing cost and improving speed of fabrication. By contrast, such molded products can be made using the microwave sintering apparatus and method set forth in the present disclosure. The particulate materials are tamped into a cavity at a desired packing density and configuration without requiring any extremely high pressures. The cavity is formed in a tube of material which is transparent to microwave radiation. This transparent tube is then positioned in the microwave cavity of the sintering apparatus. Sintering occurs at a more rapid temperature increase, yet is consummated at a lower maximum temperature level. The former feature minimizes migration of elements such as cobalt between regions or components of the article of manufacture. The latter feature reduces the possibility of high temperature induced physical or chemical damage to components of the device. Moreover, the grain size within the solid part of the device does not grow as normally occurs in a conventional sintering process. Improved hardness and chip resistance is obtained with a smaller grain structure in the molded part. The alloy sinters the entire particulate mass in the mold to thereby furnish a wear part. Examples of this will be given below.
The particulate or green material is shaped at room or ambient temperature in a mold, a preliminary process called xe2x80x9ccold pressingxe2x80x9d. The tamped or pressed particles are shaped to the desired configuration by a low cost cavity or mold. The mold need not be a high pressure mold. If the particles are sufficiently self adhesive, the particles can be precast by low pressure compaction into the desired shape and then sintered. If crumbling of the precast occurs, a sacrificial adhesive material such as wax can be mixed with the particles prior to precasting. During sintering, this sacrificial material is driven by heat from the precast. As an alternate to precasting, the green material can be formed in the low cost, microwave transparent mold can be exposed to the microwave field to sinter the mold contents.
By the use of the manufacture process of the present invention, it is possible to prepare new drill bit inserts at considerably lower temperature with smaller grain size, higher hardness and density. The process of the present invention also uses microwave sintering to obtain higher heating rates to form better PDC clad inserts. It has been found that for the microwave frequency range used and at room temperature, green materials used in the manufacture of wear inserts and the like are primarily reflective but still somewhat absorptive of microwave radiation. When exposed to microwave radiation, this partial absorption results in an initial heating of the material which, in turn, increases the dielectric constant of the material which, in turn, further increases the absorptiveness of the material which, in turn, results in further heating of the material. This xe2x80x9cbootstrapxe2x80x9d heating process terminates when the temperature of the material is elevated to a value at which the material becomes completely absorptive. This concept will be discussed further, and is a major contributor to the higher heating rate of the microwave sintering process. Heating rates as high as 300xc2x0 C./minute can be obtained. Furthermore, the desired sintering can be obtained at temperatures below which components are adversely physically and chemically altered. In the process of the invention, microwave heat is generated internally within the material instead of originating from external heating sources, and is a function of the material being processed.
As a rule of thumb, the performance of the particulates with the same hardness, toughness and density improves with decrease in grain size. It is possible to achieve very small grain sizes with high hardness, toughness and density, using the microwave processes thereby improving the characteristics when compared to the conventional process. This process requires much lower temperature (less than about 1350xc2x0 C.) than conventional sintering techniques (around 1500xc2x0 C.).
Using the apparatus described, the composite insert is placed within the microwave cavity and exposed to microwave radiation at preferably a set frequency. At this frequency and at room temperature, it has been found that the components of the insert are reflective to the microwave radiation. This is in contrast to green materials which have been found to be at least partially absorptive of the microwave radiation at room temperature. Heat from an external source is therefore optionally applied to the insert until the temperature of the insert is increased above the threshold of partial absorption or, microwave heating will suffice. At this temperature, the previously described bootstrap heating of the insert is initiated. That is, the dielectric constant of the insert begins to increase rapidly, resulting in a rapid increase in absorption of microwave energy, which in turn results in the rapid heating of the composite insert. The desired sintering temperature is rapidly reached once the insert becomes absorptive. Using this methodology, heating rates as high as 300xc2x0 Centigrade (C) per minute are obtained, thereby allowing a desired annealing temperature of perhaps 1200xc2x0 C. to be reached in only four minutes, at which time cooling can begin. Migration of alloy metal such as cobalt is negligible during these time intervals as will be discussed subsequently. Furthermore, grain size growth is held to a minimum. Finally, exposing the insert to the maximum sintering or annealing temperature for such a short period of time cause no damage, such as oxidation, to the PDC crown.